Rotating polygon scanning mirrors are used in laser printers to provide a “raster” scan of the image of a modulated laser light source across a moving photosensitive medium, such as a rotating drum. Such a system requires that the rotation of the photosensitive drum and the rotating polygon mirror be synchronized so that the beam of light (laser beam) sweeps or scans across the rotating drum in one direction as a facet of the polygon mirror rotates past the laser beam. The next facet of the rotating polygon mirror generates a similar scan or sweep, which also traverses the rotating photosensitive drum but provides an image line that is spaced or displaced from the previous image line.
There have also been prior art efforts to use a less expensive flat mirror with a single reflective surface to provide a scanning beam. For example, a dual axis or single axis scanning mirror may be used to generate the beam sweep or scan instead of a rotating polygon mirror. The rotating photosensitive drum and the scanning mirror are synchronized as the photoresistive medium or drum rotates in a forward direction to produce a printed image line on the medium that is parallel with the modulated beam scan or sweep generated by the pivoting mirror and orthogonal to the movement of the photosensitive medium.
Single axis scanning mirrors may also be used to provide a resonant scan for use with a printer. However, the return scan or sweep will traverse a path on the moving photosensitive medium (i.e., typically a rotating drum), that is at an angle with the image line printed during the forward sweep. Consequently, most prior art uses of a single axis resonant mirror in a printer required that the modulation of the reflected light beam be interrupted as the mirror made the return sweep or completed its cycle. The modulated beam was turned on again as the beam started scanning in the original or forward direction. It has been discovered, however, that at sufficiently high print speeds, both the forward and reverse sweep may be used without orthogonal adjustments.
Texas Instruments presently manufactures torsional dual axis and single axis pivoting MEMS mirrors fabricated out of a single piece of material (such as silicon, for example) typically having a thickness of about 100–115 microns. The dual axis layout may, for example, consist of the mirror surface being supported on a gimbal frame by two silicon torsional hinges, whereas a single axis mirror is supported directly by a pair of torsional hinges.
The scanning mirror surface may be of any desired shape, although an elliptical shape having a long axis of about 4.0 millimeters and a short axis of about 1.5 millimeters is particularly useful. Such an elongated ellipse-shaped mirror is matched to the shape at which the angle of a light beam is received. The gimbal frame used by the dual axis mirror is attached to a support frame by another set of torsional hinges. These mirrors manufactured by Texas Instruments are particularly suitable for use as the scanning engine for high-speed laser printers and visual display.
Color printers typically combine four (4) different modulated beam scans (e.g., one black and three selected primary colors) that print on a photosensitive medium such as a rotating drum. Since the four scans for one particular line must be properly aligned if the color printing is to be satisfactory, it is important that the four beam scans and, consequently the rotating or pivoting mirrors, run at substantially the same precise speed. This is not a problem with the rotating polygon mirrors but is a difficult problem for resonant torsional hinged mirrors. The difficulty results from the fact that, although the resonant mirrors are designed to be identical, there will be oscillating frequency differences between about 3% to 5% for a 2 kHz resonant frequency mirror or about 80 Hz.
Such differences in the resonant frequency for single mirror printers may still provide a printed page that is well within all tolerance without any adjustments. Further, even if an adjustment is required, simply adjusting or calibrating the speed of the rotating drum to the pivoting speed of the oscillating single mirror is simple and straightforward. However, if four oscillating mirrors each have a different resonant frequency, the speed of the rotating photosensitive drum can only be synchronized or adjusted with respect to one of the four resonant mirrors. The stringent alignment requirements of color printers simply make this unacceptable. Mechanisms in the paper path for compensating for different scan speeds could be used but would significantly increase the cost of the printer.
Therefore, it would be advantageous to use inexpensive resonant scanning mirrors with presently available inexpensive paper path mechanisms to produce high quality color printing.